The mechanical strength of a brittle material is controlled by three principal variables: the severity of preexisting flaws, the crack propagation resistance of the material, and the magnitude of residual and functional stresses. Thermal tempering is known to strengthen commercial glasses by inducing compressive stress in surface regions. However, the combined effect of tempering and incompatibility stress on the strength of dental ceramics as a function of design and flaw characteristics has not been analyzed previously. The long-range objective of this research program is to optimize the "margin of safety" of metal-ceramic and all- ceramic structures by controlling the distribution of stresses which develop because of tempering, specimen design, contraction coefficient differentials, structural flaws, applied external forces, and thermal history. The specific aims of this study are: 1) to test the hypothesis that thermal tempering by forced convective cooling in air progressively enhances the strength of castable ceramic, aluminous porcelain, conventional feldspathic porcelain, and highly-crystalline feldspathic porcelain as the thickness of the ceramic is increased because of larger temperature gradients which are produced during cooling, 20 to optimize stress-distribution profiles for layered ceramic structures as a function of tempering medium (air, silicone oil, and oil-water emulsions), contraction coefficient differentials, and externally applied forces based on the superposition principle for stresses, 3) to test the hypothesis that neither a positive contraction mismatch (alphaM>alphaC) nor a negative contraction coefficient (alphaM<alphaC) between metal (M) and ceramic (C) is a principal cause of marginal or generalized distortion of ultrathin metal copings of constant marginal length, 4) to test the hypothesis that a contraction mismatch can cause distortion of crowns with localized, overextended margins, 5) to test the hypothesis that feldspathic porcelain can be characterized as a linear viscoelastic material, 6) to develop a viscoelastic model and to define conditions of "thermal compatibility" for five clinically successful metal-ceramic systems based on experimental measurements and analytical computations of transient stress (during cooling) and residual stresses (which remain at room temperature), and 7) to characterize the relative "margin of safety" of ceramic crown materials and to test the hypothesis that the failure of a castable glass-ceramic is controlled more by the size of surface flaws than by the thickness of the ceramic structure. Experimental methods will employe a dilatometer to determine thermal contraction data, a bending-beam viscometer to measure midspan deflections (due to incompatibility stress) of metal-ceramic strips, a microhardness tester for inducing cracks in ceramic surfaces, mechanical testing machines to measure the viscoelastic behavior and relative strength of ceramic specimens, and computers to perform finite element analyses of thermal and mechanical stresses. The results of these studies should serve as the basis for optimizing the design and thermal processing conditions which can enhance the long-term performance of ceramic-based restorations.